I wrote an example of serving a dynamic image which is on the wiki, there is an example in the seamspace example (you'll have to get the cvs version of seam for that), a4j includes a tag for this I believe.
I think it is a bit more complicated. Here is what I have to provide.
Users can upload images to the application. They are then provided a link, which they can use within their E-Mails, or other Forums, to post references to the images.
The things users can upload do not necessarily need to be images. It could be zip files, or videos as well. Do you know what I mean?
So my question is really how would I direct the user on a certain screen, and then decide upon that screen, what Item the user wants, then set the content type, and deliver the content.
It sounds like you are on the right track with that design. Take a look at some of examples I mentioned which should give you some implementation ideas.